This application claims the benefit under 35 U.S.C. xc2xa7119 of Japanese Patent Application Nos.2000-398469 and 2001-349162, the abstracts of disclosure of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a worm wheel for use in reduction gears and the like as well as to a machine and a method for forming the same.
2. Description of Related Arts
In general, it is very difficult to form a worm wheel from a synthetic resin because the worm wheel, which is assembled with a worm shaft in a worm gear mechanism, has a lead angle at its teeth and is formed with recessed surfaces at bottom lands.
Hence, it is a general practice to use a hobbing or milling machine for shaping the worm wheel.
The fabrication of the worm wheel using a machine tool is costly because it involves a very difficult operation for transforming a gear shape into a shape to be hobbed. In addition, the hobbed piece requires deburring.
If the worm wheel is formed from the synthetic resin, there may be another problem that the presence of a parting line on a surface of action of the tooth will affect contact gear accuracies.
It is an object of the invention to provide a worm wheel formed from the synthetic resin at low costs and featuring high contact gear accuracies as well as to provide a method and machine for forming the same.
In one preferred aspect of the invention for achieving the above object, there is provided a worm wheel comprising a tooth having a tooth crest, and a bottom land formed at a bottom of a tooth space between adjoining teeth, the bottom land including a recessed surface, wherein at least the teeth are formed by injection molding a synthetic resin, and wherein the tooth crest includes a trace formed along a parting line as a result of the injection molding process.
The invention achieves high contact gear accuracies because the trace of the parting line does not exist on the surface of action of the tooth. The invention accomplishes a lower material cost than the method of forming the worm wheel by cutting and also negates the need for deburring which is necessary in the cutting process. This leads to the reduction of the manufacturing costs. The invention further offers an advantage of increasing the wear resistance of the worm wheel by virtue of a close-grained skin layer formed on the surface of the formed teeth.
In another preferred aspect of the invention, there is provided a forming machine comprising a stationary mold, a movable mold movably provided relative to the stationary mold, a stationary mold plate disposed at the stationary mold, a movable mold plate disposed at the movable mold, recesses formed in opposite surfaces of the stationary mold plate and the movable mold plate, a core disposed in the recess of the movable mold plate, a plurality of slide cores allowed to move radially of the core as arranged radially about the core, diameter decreasing means for simultaneously moving the slide cores radially inwardly relative to the core at closure of the movable mold, and diameter increasing means for simultaneously moving the slide cores radially outwardly relative to the core at opening of the movable mold. Each slide core includes a block shaped like a fan in plan, the block including a tooth crest forming face and a tooth-space forming projection formed centrally of a width of the tooth crest forming face. During the closure of the mold, adjoining slide cores are in intimate contact at a respective lateral side of their blocks while defining the parting line between their tooth crest forming faces.
According to this embodiment, the forming step is carried out with the mold closed in a manner that the plural slide cores intimately contact each other at the lateral sides of their blocks. Hence, the parting line formed in correspondence to a clearance between adjoining slide cores exists on the tooth crest of the tooth of the formed worm wheel but not on the surface of action of the tooth. Therefore, the contact gear accuracies are not decreased. The worm wheel featuring high contact gear accuracies can be formed from the synthetic resin at low manufacturing costs. In addition, the formed article need not be forcibly removed because the diameter defined by the slide cores is increased when the mold is opened.